Why would a static nested interface be used in Java?

The question has been answered, but one good reason to use a nested interface is if its function is directly related to the class it is in. A good example of this is a Listener. If you had a class Foo and you wanted other classes to be able to listen for events on it, you could declare an interface named FooListener, which is ok, but it would probably be more clear to declare a nested interface and have those other classes implement Foo.Listener (a nested class Foo.Event isn't bad along with this).

Member interfaces are implicitly static. The static modifier in your example can be removed without changing the semantics of the code. See also the the Java Language Specification 8.5.1. Static Member Type Declarations

An inner interface has to be static in order to be accessed. The interface isn't associated with instances of the class, but with the class itself, so it would be accessed with Foo.Bar, like so:

public class Baz implements Foo.Bar {
   ...
}

In most ways, this isn't different from a static inner class.

Jesse's answer is close, but I think that there is a better code to demonstrate why an inner interface may be useful. Look at the code below before you read on. Can you find why the inner interface is useful? The answer is that class DoSomethingAlready can be instantiated with any class that implements A and C; not just the concrete class Zoo. Of course, this can be achieved even if AC is not inner, but imagine concatenating longer names (not just A and C), and doing this for other combinations (say, A and B, C and B, etc.) and you easily see how things go out of control. Not to mention that people reviewing your source tree will be overwhelmed by interfaces that are meaningful only in one class.So to summarize, an inner interface enables the construction of custom types and improves their encapsulation.

class ConcreteA implements A {
 :
}

class ConcreteB implements B {
 :
}

class ConcreteC implements C {
 :
}

class Zoo implements A, C {
 :
}

class DoSomethingAlready {
  interface AC extends A, C { }

  private final AC ac;

  DoSomethingAlready(AC ac) {
    this.ac = ac;
  }
}

To answer your question very directly, look at Map.Entry.

Map.Entry

also this may be useful

Static Nested Inerfaces blog Entry

Typically I see static inner classes. Static inner classes cannot reference the containing classes wherease non-static classes can. Unless you're running into some package collisions (there already is an interface called Bar in the same package as Foo) I think I'd make it it's own file. It could also be a design decision to enforce the logical connection between Foo and Bar. Perhaps the author intended Bar to only be used with Foo (though a static inner interface won't enforce this, just a logical connection)

If you will change class Foo into interface Foo the "public" keyword in the above example will be also redundant as well because

interface defined inside another interface will implicitly public static.

In 1998, Philip Wadler suggested a difference between static interfaces and non-static interfaces.

So far as I can see, the only difference in making an interface non-static is that it can now include non-static inner classes; so the change would not render invalid any existing Java programs.

For example, he proposed a solution to the Expression Problem, which is the mismatch between expression as "how much can your language express" on the one hand and expression as "the terms you are trying to represent in your language" on the other hand.

An example of the difference between static and non-static nested interfaces can be seen in his sample code:

// This code does NOT compile
class LangF<This extends LangF<This>> {
    interface Visitor<R> {
        public R forNum(int n);
    }

    interface Exp {
        // since Exp is non-static, it can refer to the type bound to This
        public <R> R visit(This.Visitor<R> v);
    }
}

His suggestion never made it in Java 1.5.0. Hence, all other answers are correct: there is no difference to static and non-static nested interfaces.

In Java, the static interface/class allows the interface/class to be used like a top-level class, that is, it can be declared by other classes. So, you can do:

class Bob
{
  void FuncA ()
  {
    Foo.Bar foobar;
  }
}

Without the static, the above would fail to compile. The advantage to this is that you don't need a new source file just to declare the interface. It also visually associates the interface Bar to the class Foo since you have to write Foo.Bar and implies that the Foo class does something with instances of Foo.Bar.

A description of class types in Java.